US3182213A - Magnetohydrodynamic generator - Google Patents

Magnetohydrodynamic generator Download PDF

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US3182213A
US3182213A US114120A US11412061A US3182213A US 3182213 A US3182213 A US 3182213A US 114120 A US114120 A US 114120A US 11412061 A US11412061 A US 11412061A US 3182213 A US3182213 A US 3182213A
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Richard J Rosa
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    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02KDYNAMO-ELECTRIC MACHINES
    • H02K44/00Machines in which the dynamo-electric interaction between a plasma or flow of conductive liquid or of fluid-borne conductive or magnetic particles and a coil system or magnetic field converts energy of mass flow into electrical energy or vice versa
    • H02K44/08Magnetohydrodynamic [MHD] generators
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02KDYNAMO-ELECTRIC MACHINES
    • H02K44/00Machines in which the dynamo-electric interaction between a plasma or flow of conductive liquid or of fluid-borne conductive or magnetic particles and a coil system or magnetic field converts energy of mass flow into electrical energy or vice versa
    • H02K44/08Magnetohydrodynamic [MHD] generators
    • H02K44/18Magnetohydrodynamic [MHD] generators for generating AC power

Description

.51 0-11 SR SEARCH RDM FIFEKSOZ 53R 3l82213 y 4,1965 R. J. RosA- 3,182,213

MAGNETOHYDRODYNAMIC GENERATOR Fil une 1. 1961 4 Sheets-Sheet LOA D PIRRIOR ART GENERATOR l +ls RICHARD J. ROSA INVENTOR.

QMWDW WMKQM ATTORNEYS May 4, 1965 J. ROSA MAGNETOHYDRODYNAMIC GENERATOR 4 Sheets-Sheet 2 Filed June 1, 1961 TIME EQUILIBRIUM VALUE TIME RICHARD J. ROSA IN V EN TOR MZ/wD/P ATTORNEYS y 4, 1965 R. J. ROSA 3,182,213

MAGNETOHYDRODYNAMIC GENERATOR 1 Filed June 1. 1961 4 Sheets-Sheet 3 llllQllOillllll 4 t TIME TIME 0 CURRENT 0 CURRENT 0 CURRENT 0 CURRENT t TIME 2-, t, TIME o VOLTAGE o VOLTAGE TIME TIME 5 INVENTOR.

"MM/P- A Wm 3.

ATTORNEYS y t, RICHARD J. ROSA United States Patent 3,182,213 MAGNETOHYDRODYNAMIC GENERATOR Richard J. Rosa, Reading, Mass., assignor to Avco Corporation, Cincinnati, Ohio, a corporation of Delaware Filed June 1, 1961, Ser. No. 114,120 30 Claims. (Cl. 310-11) The present invention relates to a means for and method of utilizing the Hall effect in electrical equipment and, more particularly, in magnetohydrodynamic (hereinafter abbreviated MHD) generators. For convenience, the present invention is described with particular reference to MHD generators, which generate power by movement of electrically conductive fluid relative to a magnetic field, but is not limited to such applications.

MHD generators usually employ an electrically conductive working fluid from a high temperature, high pressure source. From the source, the fluid flows through the generator with which is associated a magnetic field and electrodes between which a flow of current is induced by movement of the fluid relative to the field. The fluid exhausts to a sink, which may simply be the atmosphere, or in more sophisticated systems, may comprise a recovery system including pumping means for returning the fluid to the source. The working fluid may comprise a high temperature, high pressure gas, such as helium or argon, to which is added about 1% sodium, potassium or cesium to promote ionization and hence electrical conductivity. The gas is composed of electrons, positive ions, neutral atoms, and sub-atomic particles and may, for convenience, be termed plasma.

If there is a current flow through a material or plasma perpendicular to a magnetic field, an electric field will be generated which is perpendicular both to the current and the field. This phenomenon, called the Hall effect, arises because of the force of the magnetic field on a moving charge. Such an electric field is commonly referred to as a Hall potential, and the current flow resulting from the Hall potential is commonly referred to as a Hall current. Thus, as plasma flows through the generator in the presence of an electric field and a magnetic field oriented at right angles to the electric field, curved movements of charged particles occur under the influence of both fields. By virtue of such movements, separation of negative and positive charges occurs in the plasma, resulting in a substantial potential gradient, or Hall potential, along the length of its flow. Under the influence of the Hall potential, Hall currents may circulate longitudinally through the plasma if a closed circuit is otherwise available. The Hall currents oppose direct flow of current through the plasma between the electrodes and constitute a serious loss of operating efficiency.

The idea of using the Hall potential in a generator in itself is not broadly new. The Karlovitz et al. Patent, 2,210,918, which issued on August 13, 1940, entitled Process for the Conversion of Energy in Apparatus for Carrying Out the Process, describes an early form of Hall current generator. In patent application Serial Number 860,973, filed December 21, 1959, of which I am a co-inventor, there is described an MHD generator utilizing pairs of oppositely disposed, segmented electrodes which may be connected to separate loads for preventing the flow of Hall currents in the generator. Because of the fact that no continuous path is provided longitudinally through the electrodes parallel to the direction of plasma flow, Hall currents cannot form within the plasma. The segmentation of the electrodes in eflect completely breaks the path of Hall current flow. In this way, losses associated with Hall currents are eliminated and improved over-all operation is obtained. In my patent application Serial Number 18,033, filed March 28, 1960, there is described an entirely different arrangement 3,182,213 Patented May 4, 1965 comprising an improved Hall current generator having opposed pairs of electrodes wherein the load circuit is connected between the terminal electrodes, i.e., the first and last electrodes along the length of the generator duct. Each pair of electrodes intermediate the terminal electrodes are electrically interconnected in a specified manner to increase the Hall potential.

From the preceding discussion it may readily be seen that although the existence of a Hall potential has been utilized in special cases to generate current, the existence of Hall currents has heretofore been considered undesirable, particularly in the usual form of MHD generator.

Thus, the prior art teaches that MHD generators must be designed to create a Hall potential whereby the current flow resulting therefrom may be supplied to a load, or in the alternative, that the flow of Hall currents be prevented in all cases and that the current flow resulting from conventional MHD generator action be supplied to a load.

The present invention is based on the concept that the power output of an electrical generator may be simply and advantageously controlled, as distinguished from creating or generating the power output, by utilizing Hall currents in a manner contrary to the teaching of the prior art. An example of my utilization of Hall currents contrary to the teaching of the prior art is the provision of an MHD generator wherein Hall currents are selectively prevented and permitted to flow.

One embodiment of the present invention is directed to means for and method of controlling the output of the aforementioned types of generators, and another embodiment is directed to means for and method of generating alternating current in an MHD generator.

Briefly described, a novel MHD generator in accordance with the present invention comprises a duct and a magnetic field normal to the axis of the duct. Movement of the plasma through the duct and the field induces an electromotive force between opposed electrodes that are interconnected to accommodate circulation of current (conduction current) transversely of both the magnetic field and the direction of plasma flow. Switching means is associated with the terminal electrodes to permit circulation of Hall current longitudinally through the plasma. To facilitate identification of this switching means, it will be termed, for convenience, a Hall current switch or switches. Additional switching means is also associated with the electrodes intermediate the terminal electrodes to interrupt current flow between these electrodes. To facilitate identification of this additional switching means, it will be termed, for convenience, a load switch or switches. If three or more of the intermediate elec trodes are serially interconnected, one load switch connected in series with these electrodes may be suflicient. If pairs of oppositely disposed electrodes intermediate the terminal electrodes supply separate loads, a load switch may be connected in series with each load. Thus, assume that pairs of oppositely disposed electrodes intermediate the terminal electrodes supply separate loads, the load switches associated with the loads are closed and the Hall current switch associated with the terminal electrodes is open, i.e., full load current is being delivered to separate loads by an MHD generator with segmented electrodes, and the flow of Hall currents is prevented. When the Hall current switch is closed, Hall current, which could not otherwise flow, is now permitted to flow longitudinally through the generator. The flow of Hall current reduces the conduction current or current normal to the gas flow and coupled to the loads. By reason of the reduction in the conduction current flowing through the loads, the load switches may now be opened with little or no arcing. Opening of the load switches will reduce the conduction current to zero. Reduction of the conduction current to zero in turn reduces the Hall current to zero, whereafter the Hall current switch may be opened without breaking a current. Thereafter, full power may be again delivered to the loads when the load switches are again closed. Thus, an MHD generator may be taken off the line by the simple expedient of mostly closing a switch rather than by the conventional procedure of breaking one or more high power circuits.

The arrangement described immediately hereinabove in accordance with the present invention is not only quite simple and effective but may be used, for example, to take an MHD generator on or 01f the line, control its output current, or vary its output current to provide a wave form that approaches that of a sine wave.

In view of the foregoing, it will be apparent that a broad object of the present invention is to provide an improved electric generator.

Another object of the present invention is to provide a means for and method of controlling the flow of electric current within electrical equipment.

Another object of the present invention is to provide a means for and method of taking electrical equipment on and off the line.

A further object of the present invention is to provide an MHD generator wherein the Hall effect is utilized to control the generator.

A still further object of the present invention is to provide an MHD generator with segmented electrodes which are interconnected to permit variation or interruption of the output current of the generator in a simple, economical, and efficient manner.

Yet another object of the present invention is to provide an MHD generator having pairs of opposed electrodes, each connected through a load switch to a load, the terminal electrodes of which are connected in circuit with a Hall current switch.

The novel features that I consider characteristic of my invention are set forth in the appended claims; the invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in conjunction with the accompanying drawings, in which:

FIGURE 1 is a diagrammatic representation of a conventional MHD generator;

FIGURE 2 is a diagrammatic representation of a novel arrangement in accordance with the teaching of the present invention;

FIGURE 3 is a graphic representation of the manner in which the load and Hall current varies for the arrangement illustrated in FIGURE 2;

FIGURE 4 is a diagrammatic representation of a modification of the arrangement illustrated in FIGURE 2;

FIGURE 5 is a graphic representation of the manner in which the Hall current varies for the arrangement illustrated in FIGURE 4;

FIGURE 6 is a diagrammatic representation of another embodiment of the present invention;

FIGURE 7 is a graphic representation of the manner in which the currents and voltages vary for the arrangement illustrated in FIGURE 6 when both the load and Hall current switches are actuated;

FIGURE 8 is a graphic representation of the manner in which the currents and voltages vary for the arrangement illustrated in FIGURE 6 when only the Hall current switch is actuated;

FIGURE 9 is a diagrammatic representation of another embodiment of the present invention for providing an AC. output; and

FIGURE 10 is a cross sectional view taken on line 10-10 of FIGURE 9.

A knowledge of the general principles of MHD generators Will promote an understanding of the present invention. For this reason, there is shown in FIGURE 1 a schematic of a prior art MHD generator. As illustrated in that figure, the generator comprises a tapered duct, generally designated 1, to which high temperature, high pressure electrically conductive plasma is introduced, as indicated by the arrow at 2, and from which it exhausts, as indicated by the arrow at 3. The pressure at the exit of the duct is lower than at its inlet; and for this reason, the plasma moves at high velocity through the duct, as indicated by the arrow at 4. By properly choosing the pressure differential and the shape of the duct, the plasma can be made to move through the duct at substantially constant velocity which is desirable, although not necessary to the operation of the generator. Surrounding the exterior of the duct is a continuous electrical conductor in the form of a coil 15 to which a unidirectional electrical current may be supplied from any conventional source or from the generator itself. Flow of electrical current through the coil establishes a magnetic flux through the duct, perpendicular to the direction of plasma flow and the plane of the paper.

Within the duct are provided opposed electrodes 6 and 7. These electrodes may extend along the interior of the duct parallel to the direction of plasma movement and may be positioned opposite one another on an axis perpendicular to both the direction of plasma movement and the magnetic flux. High velocity movement of the electrically conductive plasma through the magnetic field induces a unidirectional between the electrodes, such as indicated by the arrows at 8. The electrodes 6 and 7 are connected by conductors 9 and 10 to a load 11 through which electrical current flows under the influence of the E.M.F. induced between the electrodes.

Since electrons are lighter than ions and hence have a higher mobility, they will, in general, carry most of the current in an MHD generator. Since the forces exerted by the magnetic field are exerted on the current carriers, the electrons naturally experience most of the forces arising from their movement in the field.

As already mentioned, an electron current or conduction current is induced between the electrodes by the cross product of the velocity of the plasma and the magnetic field. The magnetic field acts on the current, creating a force tending to retard motion of the electrons longitudinally down the duct with the rest of the plasma. The ions, on the other hand, being much greater in mass than the electrons, only experience small forces as they move in the magnetic field and tend to be carried downstream with the plasma. Thus, a separation of charges occurs, resulting in the creation of an electric field longitudinally of the duct.

As pointed out hereinbefore, this longitudinal field may be called the Hall potential since the phenomena involved are similar to those giving rise to the so-called Hall effect observed some time ago in solid conductors. As used in the claims Hall potential means the aforementioned electric field substantially longitudinally of the duct and parallel with the direction of plasma flow which results from the aforementioned separation of charges which in turn result from the flow of conduction current.

The forces, acting on the electrons, are transmitted by them to the rest of the plasma particles by collisions. Further, the movement of plasma particles is retarded by collision with the ions which are held by the electric field existing between them and the upstream electrons. In overcoming the forces resulting from collisions with the ions and electrons, the plasma does work. This is as would be expected in a device for generating electrical power.

In FIGURE 2 there is shown diagrammatically an MHD generator in accordance with the present invention comprising a divergent duct 20 to which is supplied a high temperature, high pressure plasma 21. A magnetic field coil, indicated schematically by phantom lines at 22, is associated with the duct 20 and provides a magnetic field perpendicular to the plane of the paper and transverse of the plasma stream. In FIGURE 2, however, it will be noted that the electrodes are segmented, i.e., the electrodes at each side of the duct 20 comprise separate electrically insulated segment-s designated 23a, 23b, 23c, and 23d, and 24a, 24b, 24c, and 24d. It will be noted that each of the electrode segments of each group is positioned in side-by-side relationship, the groups of segments on opposite sides of the duct defining paths for current flow normal to the direction of both the plasma stream and the magnetic field flux. For convenience, this current will be referred to as the conduction cur rent. As used in the claims conduction current means the aforementioned current flow substantially normal to the direction of both the plasma stream and the magnetic field. In the generator shown in FIGURE 2 the conduction current is the load current. In other types of generators this may not be true.

Attention is now directed to the electrical connection of electrode segments 23a and 24a. A load, such as, for example, an inverter 25a, is connected through a load switch 26a to the electrode segments 23a and 24a. Thus, when the load switch 26a is closed, current may be supplied to the inverter 25a and when the load switch 26a is open, the circuit to the inverter 25a will be broken. The inverter, which may be of a conventional type, is connected to the primary winding 27a of a multi-winding transformer, generally designated 28, having a common secondary 29 for delivering AC. power.

In a similar manner, electrode segments 23b and 24b are connected through another load switch 26b to another inverter 25b, having a transformer primary winding 27b. The transformer primary winding 27b is also coupled with the seconday winding 29 in time-phase relationship with the primary 27a. The other opposed pairs of electrode segments are similarly connected to separate load switches and inverters which are coupled to the secondary winding 29, as illustrated. A Hall current switch 31 is connected to electrode segments 23a and 23d. The segmentation of the electrodes in effect completely breaks the path of the Hall current flow. Thus, when the Hall current switch 31 is open, no continuous path is provided longitudinally through the electrodes parallel to the direction of plasma flow, hence Hall currents cannot form within the plasma. In this way, the losses associated with Hall currents are eliminated so long as the Hall current switch 31 remains open. When the Hall current switch 31 is closed, a closed circuit is provided through which Hall currents may flow during normal operation of the generator. The flow of Hall currents decreases direct flow of the conduction current through the plasma between the electrodes and for this reason is normally considered to constitute a serious loss of operating efiiciency and, therefore, to be undesirable.

It may also be noted at this point that if the conduction current cannot flow, then the electrons in the plasma will not be retarded, and the Hall potential will not appear. Obviously, if the Hall potential does not appear, Hall currents also cannot appear.

In order to appreciate the manner in which the present invention operates, assume that the Hall current switch 31 is open and the load switches 26a26d are closed. Under these circumstances, power will be delivered to the inverters or any other suitable load, as in a normal MHD generator with a segmented output. When the Hall current switch 31 is closed, Hall current can flow through electrode 23d, the Hall current switch 31, electrode 23a, and the plasma 21 between electrode 23a and electrode 23d. As previously explained in connection with FIGURE 1, this flow of Hall current reduces the conduction current flowing between opposed pairs of electrodes. If the Hall current is of sufiicient magnitude, it will reduce the conduction current to zero. By reason of the reduced current flowing therethrough, the load switches 26a-26d may now be opened with little or no arcing. If the conduction current is at some finite value,

opening of the load switches 26a-26d reduces the conduction current to zero. As previously explained, this in turn reduces the Hall current to zero, whereafter the Hall current switch 31 may be opened without breaking a current. -Full load current will again be delivered to the inverters or loads when the load switches 26a-26d are closed subsequent to the opening of the Hall current switch 31. As will now be evident, the present invention in a new and novel manner permits the variation and/or interruption of the power output of an MHD generator mostly by closing switches rather than opening them in conventional manner. Bearing in mind that it is much more difficult to break a high power circuit than to make it and that it is much less difficult to break a low power circuit than a high power circuit, it will immediately be apparent that the present invention is quite useful in controlling the power output of MHD generators.

That the Hall current may be advantageously utilized to control the output of an MHD generator may be seen from the following discussion of the equations that illustrate the principles involved. It should be noted at this point, however, that the following equations are theoretical in that they are for a uniform plasma. A uniform plasma is presumed because it is believed that the underlying principles of the present invention can most clearly be explained and understood on this basis. However, as is always the case, theory can only be approached in actual practice. Thus, in actual practice the plasma in an MHD generator will not be uniform and, hence, in this respect, the equations are only approximately correct.

The Hall current jx which flows parallel to the plasma stream may be determined from the general equation and the conduction current jy which flows transverse of the plasma stream may be determined from the general equation +OJ2T2['ILB where a=the scale of conductivity of the plasma w=the electron cyclotron frequency in radians/ sec.

T=the electron mean free time between collisions with plasma particles in seconds B=the magnetic field strength Ey=the potential between electrodes transverse of the plasma stream Ex=the Hall potential longitudinally through the plasma stream u=the macroscopic velocity of the plasma stream The values for w and 1- for any given plasma can be calculated by using the principles set forth in Physics of Fully Ionized Gases by Lyman Spitzer, Jr., Interscience Publishers, Inc., 1956, and other standard reference works.

Assuming no leakage effects of current through boundary layers or due to other inhomogeneities in the plasma or magnetic field, if the Hall current switch 31 is open and the load switches 26a-26d are closed, the Hall current jx is equal to zero and the full load current jy is given by the following solution of the preceding general equations for ix and jy:

where 1 is the electrical efiiciency of the generator or, to state it another way, the fraction of work done by the plasma that is delivered to the load in the form of electrical power.

If the Hall current switch 31 is now closed, the Hall potential Ex becomes nominally equal to zero and as a result the conduction current is reduced. The reduced conduction current j"y resulting where there is a flow of Hall current is given by the equation:

At the same time that the conduction current iy is reduced the Hall current ix rises from zero to a finite value. The increased Hall current j'x may be determined from the equation If now the load switches 26a-26d are opened, the conduction current jy will, of course, drop to zero, and it can be shown from the equations for the Hall current jx and the conduction current jy that the Hall current jx also drops to zero.

Thus, it may now be apparent that when the load switches 26a-26d are opened subsequent to the closing of the Hall current switch 31, the load switches will not have to break the full load current jy. Since the Hall current jx drops to zero when the load switches are opened, the Hall current switch when it is opened does not break a current.

Attention may now be directed to FIGURE 3, which illustrates an idealized form of load and Hall current variation for the embodiment illustrated in FIGURE 2, it being assumed that there is negligible reactance in any of the circuits. As shown in FIGURE 3, if the load switches 26a-26d are closed at time t the load current, represented by the solid line, will rise to its maximum value and remain constant until the Hall current switch 31 is closed. Upon closing of the Hall current switch at time t the load current will drop to at least a low value and remain at this value until such time as the load switches are opened, such as, for example, at time t The Hall current, represented by the broken line, rises from zero at time t to a finite value upon closure of the Hall current switch 31 and, as previously indicated, drops to zero at time t when the load switches are opened. As can readily be seen from inspection of FIGURE 3, the length of time that the Hall current is permitted to flow is determined by the delay between the time when the Hall current switch is closed and the load switches are opened. Thus, the time between t and t can be made quite short, it only being necessary that the opening of the load switches be delayed until the load current has reached its minimum value which, as will be pointed out hereinafter, can be zero. When the load switches are again closed at time t.,, the cycle will be repeated.

It will be seen from the previously discussed equations that the usefulness of the present invention depends upon the value of w. Thus, by way of example, for an electrical efliciency 1; of 0.8 the rat-i of the reduced conduction current to the full load current j"y/j'y may have the following values for the given values of air:

or j'yljy 1 0. so JR) 0. 33 1o 0. 05

For a generator efiiciency n .of 0.5, the ratio of j"y/j'y has the following values for the same values of on.

Thus, from the above it may readily be seen that if mis equal to one, a reduction of only 20 to 30% in the full load current jy will be obtained. However, if M' is equal to 10, then the full load current jy will be reduced by more than a factor of 10.

In actual practice, car may be increased by increasing the magnetic field strength, decreasing the plasma density, selecting a plasma composition that results in a small elec tron collision cross section, or by a combination of any or allot the foregoing. Argon, for example, has an electron collision cross section roughly one-thirtieth of most gases.

The time that Hall current flows can be made quite short. Thus, it is only necessary that the Hall current switch be of a conventional contaotor type that can carry the maximum Hall current for the required time since it is only necessary that it make a circuit rather than break a circuit. F urthcr, if the conduction current is made to effectively drop to zero, such as, for example, in the manner hereinafter described, the load switches may also be of the contactor type, capable of continuously carrying the full load current. Consider now the most unfavorable case where, for example, for practical reasons, it may be desired that the load switches break a current of sufficient magnitude as to cause appreciable arcing. In this case so far as arcing is concerned, the load switches need only be capable of breaking a current considerably less than the full load current.

A word may also be said at this point with respect to ,short circuits. The Hall current and load switches may be actuated manually. They may obviously also be actuasted in timed relationship by conventional means, such as, for example, a motor and sensing means to actuate the motor. Thus, if the sensing means is sensitive to short circuit conditions, the generator or circuit containing the short circuit can be immediately shut down thereby eliminating the necessity of considering short circuit currents in, for example, the selection of the load switches.

Attention is now directed to FIGURE 4, which illustrates a modification of the embodiment shown in FIG- URE 2. The arrangement shown in FIGURE 4 is identical to that shown in FIGURE 2 except for the provision of a tank circuit 41 connected in series with the Hall current switch 31. The tank circuit 41 is comprised of an inductance 42 connected in parallel with a capacitor 43. Further, although it is not essential, it is preferable that a rectifying action be provided in the conduction current circuit or circuits. Such an effect may be achieved, for example, by maintaining one set of electrodes at a temperature below that at which they will emit electrons and/or forming them of a non-emissive material. Alternately, a conventional rectifier may be provided in the conduction current circuits, such as, for example, between opposite pairs of electrodes, if the load or loads do not function as a rectifier to achieve the same result.

For a sufficiently high an and a suitable choice of the values of inductance and capacitance in the tank circuit 41, a Hall current jx due to the action of the tank circuit will overshoot its equilibrium value when the Hall current switch 31 is closed. At the same time, the conduc tion current jy will momentarily try to reverse its direction of flow. Thus, if the aforementioned rectifying action is provided in the conduction current circuit, then the conduction current jy will momentarily be Zero, and the load switches may be opened at this time without breaking a current. The rectifying action is desirable because it prevents the conduction current from actually reversing, it insures essentially zero conduction current flow, and tends to increase the time that the conduction current is zero.

FIGURE 5 illustrates the manner in which the Hall current jx will vary. In FIGURE 5 it is assumed that the load switches are closed and the Hall current switch is open. The Hall current switch is closed at time t In order that the conduction current drop to zero subsequent to time t it is necessary that wr and the circuit elements be chosen such that the Hall current jx at time t be at least equal to and preferably greater than in order that the conduction current jy be reduced to zero. The required values of the circuit parameters will depend upon the circumstances under which it is desired to operate and can be found by straightforward transient circuit analysis. As has been previously pointed out, the values of tor, the inductance, and the capacitance must be selected such that the initial overshoot of the Hall current at time t cuts off or materially reduces the flow of conduction current across the generator duct. The time available for opening the load switches is determined by the relation of jx at time i to Thus, if jx at time t is just equal to essentially zero time is available to open the load switches. On the other hand, the time available to open the load switches is increased in proportion to the amount that jx at time 1 is made greater than The foregoing requires that anbe generally greater than about 4 or 5. Provision of a mgreater than about 4 or 5 greatly facilitates the design and operation of the load switches, since they may be opened under zero current conditions.

Attention is now directed to FIGURE 6, which illustrates another arrangement for reducing the conduction current to zero. The arrangement shown in FIGURE 6 is identical to that shown in FIGURE 4 with the exception of the Hall current circuit between electrodes 23a and 23d exterior of the duct. As shown in FIGURE 6, the tank circuit of FIGURE 4 is omitted, and the Hall current switch 31 of FIGURE 4 is replaced by a double pole, double throw, reversing switch 51. The electrodes 23a and 23d are connected to respectively terminals 52 and 53 of switch 51, as are switch arms 54 and 55 A capacitor 56 is connected across terminals 57 and 58 of switch 51. Terminal 61 is connected to terminal 57, and the remaining terminal 62 is connected to terminal 58 in conventional manner, such that as switch 51 is thrown from one position to the other the connection of capacitor 56 to electrodes 23a and 23d is reversed. Thus, assume that the load switches 26a-26d are closed and switch 51 is open, i.e., the Hall current circuit is open and capacitor 56 is not connected to electrodes 23a and 23d. When switch 51 is closed in one direction, capacitor 56 will be charged to the full Hall potential. Obviously, after the charge on capacitor 56 reaches the full Hall potential, no further current will flow in the Hall current circuit. If now switch 51 is thrown in the opposite direction, the connection of capacitor 56 to electrodes 23a and 23d will be reversed. Upon reversal of switch 51 the charge on capacitor 56, which is substantially equal to the Hall potential, is connected in series aiding with the Hall potential. Thus, a Hall current will momentarily be permitted to flow which may be up to twice as big as the Hall current drawn in the apparatus illustrated in FIGURE 2. For the arrangement illustrated in FIGURE 6 the value of or need not be as high as that required, for example, by the arrangement illustrated in either FIGURE 2 or FIGURE 4. Thus, if w'r is sufliciently high, such as, for example, greater than about 2, the flow of Hall current resulting from reversal of switch 51 will reduce the conduetion current jy to zero forthe same reasons discussed in connection with FIGURE 4. Also, for the reasons discussed in connection with FIGURE 4, a rectifying action in the conduction current circuits is desirable. When capacitor 56 has been charged up in the reverse direction due to the reversal of switch 51, returning switch 51 to its original position will produce another momentary cutoff of the conduction current jy. The duration of cutoif of the conduction current jy is determined by the Values of the capacitor 56, a, and the load impedances. Again, the required values of circuit parameters for a specific application can be found by straightforward transient circuit analysis.

Assuming resistive circuits, FIGURE 7 illustrates the manner in which the conduction current jy, Hall current ix, and Hall potential Ex will vary for the various positions of the Hall current switch 51 and the load switches 26a-26d discussed immediately hereinabove. Thus, upon reversal of the connection of capacitor 56 to electrodes 23a and 23d by actuation of the Hall current switch 51 from one position to another at time t the conduction current jy will tend to reverse as suggested by the negative portion of the curve jy. The presence of a rectifying action will eliminate this negative portion. Simultaneously at time t the Hall current ix will increase from zero to its maximum value and thereafter decrease at a rate determined by the charge on capacitor 56 and the magnitude of the Hall potential Ex between electrodes 23a and 23d. Also, at time t; the Hall potential Ex drops from its normal or steady state value to a negative value and thereafter increases in a positive direction. If now the load switches 26a-26d are opened at time t the load current is fixed at zero, and the Hall current ix drops rapidly as shown due to the removal of the Hall potential. The Hall current jx that flows after the load switches are opened is due to and controlled by the charge on capacitor 56. The Hall potential Ex, of course, continues to rise from a negative value toward zero, and its rate of rise is determined by the flow of Hall current.

It the load switches 26a-26d are closed at time t the conduction current jy and the Hall potential Ex rapidly rise to their steady state values. Thus, the flow of Hall current jx through capacitor 56 rapidly decreases from a value less than its value at time t The curves of voltage and current illustrated in FIG- URE 8 show the relation and manner of variation of the conduction current jy, Hall current jx, and Hall potential Ex for reversal of the Hall current switch 51 without actuation of the load switches 26a26d.

Inspection of the curve for the conduction current jy in FIGURE 8 will show that if, for example, the polarity of the conduction current is alternately reversed with respect to the load during the time between about time i and t a current output waveform may be obtained that approaches that of a sine wave. This may be accomplished, for example, by use of an electrode arrangement and output circuit described hereinafter in connection with FIGURE 9.

With reference now to FIGURE 9, there is shown an arrangement similar to that illustrated in FIGURE 6. However, several important differences should be noted. Firstly, it should be noted that the electrodes for accommodating conduction current are comprised of two groups of oppositely disposed electrodes designated generally by the numbers 71 and 72. Group 71 is comprised of electrodes 73a73c and 74a-74c and group 72 is comprised of electrodes 75a-75c and 7611-760.

Next, it should be noted that separate terminal electrodes, designated by the numbers 77, 78, and 79, comprised of a pair of oppositely disposed electrodes parallel to the magnetic field are associated with the groups of electrodes 71 and 72. The terminal electrodes 77, 78, and 79 accommodate the flow of Hall current. Directing attention to FIGURE 10, it will be noted that the terminal electrode 77 may be comprised of a pair of interconnected electrode segments 81 and 82 carried in respectively op posite sidewalls 83 and 84 of the duct perpendicular to the magnetic field. Thus, electrode segments 81 and 82 are positioned parallel to the magnetic field and perpendicular to the direction of plasma flow. Terminal electrodes 78 and 79 may be essentially identical to terminal electrode 77. Terminal electrode 77 is positioned upstream of electrodes 73a and 7411, and terminal electrode 79 is positioned downstream of electrodes 75c and 760. Terminal electrode 78, however, is positioned intermediate electrodes 73c, 74c, and 75a, 76a. Terminal electrode 78 thus separates the electrodes for accommodating conduction current into two group-s as and for the purposes hereinafter described. All of the electrodes, whether for accommodating conduction current or Hall current, are insulated from the duct. Further, the terminal electrodes may be comprised of a greater number of electrode segments than that shown or of annular rings suitably supported within and insulated from the duct. However, where a ring-type terminal electrode or its equivalent is used, it must be positioned along a plane of equal potential within the generator to prevent internal shorting of the generator. A-complete discussion of such terminal electrode configurations may be found in my patent application Serial Number 32,969, filed May 5, 1960, to which reference is made.

Next, it should be noted that the Hall current reversing switch 51 of FIGURE 6 is replaced by a single pole, double throw Hall current switch 85 having three terminals, 86, 87, and 88. Terminal 87 of the single pole, double throw Hall current switch 85 is connected to the intermediate terminal electrode 78 through a capacitor 89 and conductor 91. Terminal 86 is connected through conductor 92 to terminal electrode 77, and terminal 88 is connected through conductor 93 to terminal electrode 79. Electrodes 73a and 74a are connected through a load switch 94a to a primary winding 95a of an output circuit comprising a muiti-winding transformer, generally designated 96, having common a secondary winding 97 for delivering A.C. power. In a similar manner, electrode segments 73b and 74b are connected through another load switch 94b to another primary winding 95b. The primary winding 95]; is also coupled with the secondary winding 97 in time-phase relationship with the primary winding 95a. The other opposed pairs of electrode segments are similarly connected to separate load switches and primary windings, as illustrated. The dots associated with the primary windings 9511-95c and 9911- 990 of transformer 96 indicate reversal of the polarity of these windings. Thus, a current induced in the secondary winding 97 by primary windings 9511-950 will be opposite in polarity to that induced in the secondary winding by primary windings 9911-990. The connection of the Hall current and load switches to suitable actuating means, designated by the number 101, suggests that these switches may be actuated in timed relationship one with another.

The principle of operation of the arrangement illustrated in FIGURE 9 is similar to that described in connection with FIGURE 6. Thus, assume that load switches 9411-940 are closed, that load switches 9811-980 are open, that the Hall cunrent switch 85 is connected between terminals 87 and 88, and that there is no charge on capacitor 89. If the load switches 9811-980 are now closed, a charge will build up on capacitor 89 at the same time that the current through load switches 9811-98c increases. If the Hall current switch 85 is now thrown to its other position or terminal 86, capacitor 89 will be connected between the intermediate terminal electrode 78 and terminal electrode 77 and the charge on capacitor 89 will be connected in series aiding with the Hall potential between the aforementioned terminal electrodes. Thus, a Hall current will be permitted to flow, for example, between terminal electrodes 77 and 78, which may be up to twice as big as the Hall current that would be drawn if terminal electrodes 77 and 78 were merely short circuited. Due to the flow of Hall current between the aforementioned terminal electrodes, the conduction current between electrodes 7311-730 and 7411-74c is reduced. Load switches 94a-94lc are now opened. The load switches 946l946 may remain open, for example, for about one-half cycle of the desired period of the alternating current output. The load switches 9411-940 are then closed and a charge builds up on capacitor 89 due to the potential gradient between terminal electrodes 77 and 78. The polarity of this new charge on capacitor 89 will be reversed from that of the previous charge. After the potential on capacitor 89 has reached a suitable level, the Hall current switch is thrown from terminal 86 to terminal 88 to reduce the current flowing through load switches 9811-980. Thereafter, load switches 980-986 may be opened to repeat the cycle. The current induced in the secondary winding 97 when load switches 98a-98c are closed is out of phase with the current previously induced therein by the flow of current in primary windings 9511-950. Although a limited amount of overlap is present in the flow of current through the primary windings, it will now be evident that an alternating current output is available at the secondary winding.

In view of the preceding discussion, it will be apparent that the electrode arrangement and output circuit illustrated in FIGURE 9 may be modified to utilize the switching arrangement illustrated in FIGURE 2 or FIG- URE 4.

It will now be obvious that the present invention is subject to various modifications and may be utilized in a number of different ways. For example, the present invention may be utilized to take an MHD generator on or off the line, vary its output current, or produce alternating current. In short, it may be used to control an MHD generator. Further, separate terminal electrodes of various construction may be utilized or certain of the electrodes for accommodating conduction current may also function as terminal electrodes. Electrode arrangements for accommodating conduction current may be repeated to provide, for example, separate load circuits or an alternating current output. The load switches may be actuated in timed relationship with the Hall current switchfor certain purposes, or the Hall cunrent switch alone may be actuated. Also, the electrodes for accommodating conduction current need not necessarily be constructed or connected as shown and described herein. The present invention is equally useful with electrode arrangements, for example, as shown and described in the patent applications to which reference has previously been made. In these cases, where it is desired, for example, to take an MHD generator on or off the line, the Hall current switch would function to short out the load, contrary to accepted practice or what one would expect. Also, when the electrodes for accommodating conduction current are connected in series, the number of load switches need not necessarily equal the number of pairs of oppositely disposed electrodes.

The various features and advantages of the invention are thought to be clear from the foregoing description. Various other features and advantages not specifically enumerated will undoubtedly occur to those versed in the art, as likewise will many variations and modifications of the embodiments of the invention illustrated, all of which may be achieved without departing from the spirit and scope of the invention as defined by the following claims.

1. In combination, first means for conveying an electrically conductive fluid; means for establishing magnetic flux through said first means at an angle to the direction of flow of the buid; a plurality of discrete electrodes spaced from each other within said first means, the connection in circuit of said plurality of electrodes allowing a potential gradient within the fluid in the direction of flow; and means for selectively reducing said potential gradient to about zero.

2. In combination, first means for conveying an electrically conductvie fluid; means for establishing magnetic flux through said first means at an angle to the direction of flow of the fluid; a plurality of discrete electrodes spaced from each other within said first means, the connection in circuit of said plurality of electrodes allowing a potential gradient within the fluid in the direction of flow; and means for selectively providing substantially a short circuit of said potential gradient.

3. In combination, first means for conveying an electrically conductive fluid; means for establishing magnetic flux through said first means at an angle to the direction of flow of the fluid; a plurality of discrete electrodes spaced from each other within said first means, the flow of current through said plurality of electrodes allowing a potential gradient within the fluid in the direction of flow; and means for selectively providing substantially a short circuit of said potential gradient for controlling the current flow through said plurality of electrodes.

4. In a device for generating electric power wherein a magnetohydrodynamic generator employs an electrically conductive fluid having a potential gradient within the fluid in the direction of flow, the combination with said generator of means for selectively providing substantially a short circuit of at least a portion of said potential gradient.

5. In a device for generating electric power wherein a magnetohydrodynamic generator employs an electrically conductive fluid having a potential gradient within the fluid in the direction of flow, the combination with said generator of means including switching means for providing substantially a short circuit of at least a portion of said potential gradient.

6. In a device for generating electric power wherein a magnetohydrodynamic generator employs an electrically conductive fluid having a potential gradient within the fluid in the direction of flow, the combination with said generator of means for selectively providing substantially a short circuit external of said fluid of at least a portion of said potential gradient, said means including switching means.

7. In a device for generating electric power wherein a magnetohydrodynamic generator employs a duct for conveying a stream of electrically conductive fluid having a potential gradient in its direction of flow, means for establishing a magnetic flux through said duct normal to the direction of flow of said fluid, and opposed electrodes within said duct aligned perpendicularly to the magnetic flux and the direction of flow of the fluid, the combination with said generator of means for selectively providing external of said fluid substantially a short circuit of at least a portion of said potential gradient.

8. In a device for generating electric power wherein a magnetohydrodynamic generator employs a duct for conveying a stream of electrically conductive fluid having a potential gradient in its direction of flow, means for establishing a magnetic flux through said duct normal to the direction of flow of said fluid, and opposed electrodes within said duct aligned perpendicularly to the magnetic flux and the direction of flow of the fluid, the combination with said generator of means including first switching means for selectively providing external of said fluid substantially a short circuit of at least a portion of said potential gradient; and second switching means connected in series with certain of said opposed electrodes.

9. In a device for generating electric power wherein a magnetohydrodynamic generator employs a duct for conveying a stream of electrically conductive fluid having a potential gradient in its direction of flow, means for establishing a magnetic flux through said duct normal to the direction of flow of said fluid, and opposed electrodes within said duct aligned perpendicularly to the magnetic flux and the direction of flow of the fluid, the combination with said generator of means including first switching means for selectively providing external of said fluid substantially a short circuit of at least a portion of said potential gradient; second switching means connected in series with at least two opposed electrodes; and means for actuating said first and second switching means in timed relationship.

10. In a device for generating electric power wherein a magnetohydrodynamic generator employs a duct for conveying a stream of electrically conductive fluid having a potential gradient in its direction of flow, means for establishing a magnetic flux through said duct normal to the direction of flow of said fluid, and opposed electrodes within said duct aligned perpendicularly to the magnetic flux and the direction of flow of the fluid, the combination with said generator of means including first switching means for selectively providing external of said fluid substantially a short circuit of at least a part of said potential gradient; second switching means connected in series wih selected pairs of opposed electrodes; and means for actuating said first and second switching means in timed relationship.

11. In combination, first means for conveying an electrically conductive fluid; means for establishing magnetic flux through said first means at an angle to the direction of flow of the fluid; a plurality of discrete and opposed electrodes spaced from each other within said first means and connected in circuit, the connection in circuit of opposed electrodes establishing a potential gradient within the fluid in the direction of flow; means for selectively providing substantially a short circuit of at least a portion of said potential gradient; and switching means con nected in series with selected pairs of opposed electrodes.

12. In combination, first means for conveying an electrically conductive fluid; means for establishing magnetic flux through said first means at an angle to the direction of flow of the fluid; a plurality of discrete electrodes spaced from each other within said first means, the connection in circuit of said plurality of electrodes establish ing a potential gradient within the fluid in the direction of flow; first means for selectively providing substantially a short circuit of ditferent portions of said potential gradient; switching means connected in series with selected pairs of opposed electrodes; and second means for actuating said first means and said switching means in timed relationship.

13. In a magnetohydrodynamic generator the combination comprising: a duct for conveying a stream of electrically conductive plasma; first means for establishing a magnetic flux through said duct substantially normal to the direction of flow of the plasma; opposed electrodes within said duct and aligned perpendicularly to the magnetic flux and the direction of flow of the plasma, each of said electrodes comprising discrete segments whereby current flow substantially perpendicular to the magnetic flux and the direction of flow of the plasma between the opposed segments establishes a potential gradient within the plasma in the direction of flow; second means including switching means for causing said potential gradient to at least momentarily decrease; and circuit interrupting means connected in series with selected pairs of opposed electrodes.

14. In a magnetohydrodynamic generator the combination comprising: a duct for conveying a stream of electrically conductive plasma; first means for establishing a magnetic flux through said duct substantially normal to the direction of flow of the plasma; opposed electrodes within said duct and aligned perpendicularly to the magnetic flux and the direction of flow of the plasma, each of said electrodes comprising discrete segments whereby current flow substantially perpendicular to the magnetic flux and the direction of flow of the plasma between the opposed segments establishes a potential gradient within the plasma in the direction of flow; second means including switching means for permitting at least momentary current flow within said plasma parallel to the direction of plasma flow; and circuit interrupting means connected in series with selected pairs of opposed electrodes.

15. In a magnetohydrodynamic generator the combination comprising: a duct for conveying a stream of electrically conductive plasma; first means for establishing a magnetic flux through said duct substantially normal to the direction of flow of the plasma; opposed electrodes within said duct and aligned perpendicularly to the magnetic flux and the direction of flow of the plasma, each of said electrodes comprising discrete segments whereby conduction current flow between the opposed segments establishes a Hall potential within the plasma in the direction of flow; second means including switching means for permit-ting sufficient Hall current to flow at least momentarily to reduce conduction current; and circuit interrupting means connected in series with selected pairs of opposed electrodes to open circuit the conduction current.

16. The combination as defined in claim 15 wherein the second means additionally includes a tank circuit.

17. The combination as defined in claim 15 wherein the second means is coupled to different portions of the Hall potential.

18. The combination as defined in claim 15 wherein said second means is connected between terminal electrode segments.

19. The combination as defined in claim 15 additionally including terminal electrodes, said terminal electrodes separating the electrode segments into groups, said second means being coupled to said terminal electrodes; and means for actuating said second means and said circuit interrupting means in timed relationship.

'20. In a magnetohydrodynamic generator the combination comprising: a duct for conveying a stream of electrically conductive plasma; first means for establishing a magnetic flux through said duct substantially normal to the direction of flow of the plasma; opposed electrodes within said duct and aligned perpendicularly to the magnetic flux and the direction of flow of the plasma, each of said electrodes comprising discrete segments whereby conduction current flow between the opposed segments establishes a Hall potential within the plasma in the direction of flow; second means for permitting Hall current to flow, said second means including a source of potential and means for connecting said source of potential in series aiding across at least a selected portion of said Hall potential to reduce conduction current within said selected port-ion to about Zero; and second circuit interrupting means connected in series with selected pairs of opposed electrodes.

21. The combination as defined in claim wherein said source of potential is a capacitor and said means for connecting said source of potential in series aiding is a reversing switch.

22. The combination as defined in claim 20 wherein said second means includes a source of potential and means for connecting saidsource of potential in series aiding across different portions of said Hall potential for reducing conduction current within said dilferent portions to about Zero; and additionally including means for actuating said second means and circuit interrupting means in timed relationship.

23. In a device for generating electric power wherein a magnetohydrodynamic generator which generates electrical power by movement of electrically conductive fluid relative to magnetic field employs a plurality of discrete electrodes spaced from each other, the combination with said generator of means for selectively providing substantially a short circuit between certain of the electrodes longitudinally of the generator.

24. The method of controlling the flow of conduction current within a stream of plasma flowing through a magnetic field at an angle to the direction of flow of the plasma comprising: selectively causing current to flow longitudinally through the plasma.

25. The method of controlling the flow of conduction current within a stream of plasma flowing through a magnetic field at an angle to the direction of flow of the plasma and having a Hall potential in the direction of flow comprising: selectively permitting Hall current to flow longitudinally through the plasma.

26. The method of controlling the flow of conduction current within a stream of plasma flowing through a magnetic field at an angle to the direction of flow of the plasma and having a Hall potential in the direction of flow comprising: selectively short circuiting the Hall potential.

27. The combination as defined in claim 26 and additionally including the step of interrupting the conduction current subsequent to short circuiting of the Hall potential.

28. The method of controlling the flow of conduction current within a stream of plasma flowing through a magnetic field at an angle to the direction of flow of the plasma and having a Hall potential in the direction of flow comprising: selectively substantially short circuiting the Hall potential at least momentarily through a source of potential connected in series aiding with said Hall potential.

29. The method of controlling a magnetohydrodynamic generator having a duct through which flows a stream of electrically conductive plasma, means for establishing magnetic flux through the plasma perpendicularly to its direction of flow, and opposed segmented electrodes within the duct between which conduction current flows substantialy mutually perpendicular to the direction of plasma flow and the magnetic flux, a Hall potential normally existing within the plasma in the direction of flow comprising: substantialy short circuiting at least a portion of said Hall potential; thereafter interrupting the flow of conduction current disposed within said short circuited portion of the Hall potential; and thereafter removing said short circuit.

30. The method of controlling a magnetohydrodynamic generator having a duct through which flows a stream of electrically conductive plasma, means for establishing magnetic flux through the plasma perpendicularly to its direction of flow and opposed segmented electrodes within the duct between which conduction current flows substantially mutually perpendicular to the direction of plasma flow and the magnetic flux, a Hall potential normally existing within the plasma in the direction of flow comprising: causing a current to flow longitudinally through at least a portion of the plasma sufiicient to at least reduce conduction current in said portion; and interrupting said conduction current in said portion of the plasma when the longitudinal current flow is about maximum.

References Cited by the Examiner UNITED STATES PATENTS 2,592,970 9/60 Blackman 310-11 FOREIGN PATENTS 841,613 6/52 Germany. 1,161,079 3/58 France.

MILTON O. HIRSHFIELD, Primary Examiner.

DAVID X. SLINEY, Examiner.

Claims (1)

1. IN COMBINATION, FIRST MEANS FOR CONVEYING AN ELECTRICALLY CONDUCTIVE FLUID; MEANS FOR ESTABLISHING MAGNETIC FLUX THROUGH SAID FIRST MEANS AT AN ANGLE TO THE DIRECTION OF FLOW OF THE BUID; A PLURALITY OF DISCRETE ELECTRODES SPACED FROM EACH OTHER WITHIN SAID FIRST MEANS, THE CONNECTION IN CIRCUIT OF SAID PLURALITY OF ELECTRODES ALLOWING A POTENTIAL GRADIENT WITHIN THE FLUID IN THE DIRECTION OF FLOW; AND MEANS FOR SELECTIVELY REDUCING SAID POTENTIAL GRADIENT TO ABOUT ZERO.
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GB756662A GB987590A (en) 1961-06-01 1962-02-27 Improvements in or relating to the control of current in plasma
CH252162A CH440431A (en) 1961-06-01 1962-03-01 Method and device for controlling cross current in a plasma flow

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DE1438258A1 (en) 1968-10-03
CH440431A (en) 1967-07-31
GB987590A (en) 1965-03-31
DE1438258B2 (en) 1970-05-21

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